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How do you determine if an outlet is part of a unidentified multi-wire branch circuit? Also, does code say MWBCs need to be on double throw circuit breakers and is it permissible to link CBs with that hole in the handle?

Mazura
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How do you determine if an outlet is part of a unidentified multi-wire branch circuit?

Here are three possible scenarios:

  • There are 2 different hot wires (not a splice) and only 1 neutral wire connected to the outlet. This can be a single 240 V circuit, or it could be two 120 V circuits on a MWBC.
  • Tracing the hot wire within the panelboard. Remove the cover of the panelboard and look for the cable or conduit where the circuit enters. If that circuit has more than one hot wire (connected to more than one breaker pole) then it could be a MWBC.
  • Tracing the entire circuit. For the sake of updating outlets or other repairs, this is sometimes how MWBCs are discovered when they are not properly identified at the panel. By tracing an outlet to a splice where there are two separate hot wire colors, this reveals the presence of a MWBC.

Note that measuring voltage is NOT a valid method. It is possible to have a MWBC installed wrong, such that the hot wires have zero voltage between them. This is why the other visual search methods are necessary.

Also note that turning off a single pole breaker identified for an outlet is not a reliable method. If the outlet is part of an unidentified MWBC, then there is a shared neutral involved and it will not get disconnected by the circuit breaker.

Does code say MWBCs need to be on double throw circuit breakers.

No, it is not required but sometimes still the easiest/best solution. The technical term for this is "common trip". NEC 240.15 provides an exception for MWBC if they are strictly used for line-to-neutral loads (120 V appliances only).

is it permissible to link CBs with that hole in the handle?

Yes, using a listed handle tie that is designed for that purpose. NEC 210.4(B) requires "a means that will simultaneously disconnect" the MWBC. This requirement has been in place for decades already.

Robert Chapin
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  • @DeanMacGregor That simply isn't accurate. A MWBC can have 0 V between hot wires. Turning off one breaker won't make it safe to work on. In this case, the 0 V reading indicates a much more dangerous situation. Combine that with, say, a partial-open neutral and a ground fault or two on the same circuit and it's a recipe for disaster. – Robert Chapin Aug 24 '23 at 15:33
  • I see it as a waste of effort. In my own home, all of the MWBC outlets have only one hot wire. So this method would give nothing but false information – Robert Chapin Aug 24 '23 at 16:27
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The easy way to tell if an outlet is powered by an MWBC is to turn off the breaker(s) known to supply power to that MWBC and see if the outlet's power goes away.

The US National Fire Protection Agency's National Electrical Code does specify that all nongrounded conductors in an MWBCs must be controlled by a double-pole breaker or marked pair of linked breakers.

It's absolutely permissible to link breakers together using that hole - that's what the hole is there for. In most cases, though, it's a better idea to use a unitized double-pole breaker; that way you're likely to get the best possible match and service life between the two individual internal breakers.

Note that it is NOT permissible to link two breakers with different CURRENT ratings together in that manner - all nongrounded conductors in an MWBC must be the same size wire, and so if two linked individual breakers are used, then they MUST have the same current ratings.

TDHofstetter
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    It is also the case that a three phase MWBC must have a triple pole breaker ganged together. – wallyk Aug 17 '14 at 06:24
  • The nec does say that the link needs to be listed for the purpose but yes I always felt that a 2 (or 3) pole is better. – user24125 Aug 17 '14 at 08:51
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    @user24125 can you show me the code reference that disallows a 20A and 30A linked if#10 AWG is used for all three conductors even though I have never done that. – user24125 Aug 17 '14 at 09:03
  • What if they are improperly installed? I am dealing with many unidentified not in tandem, nor pigtailed, MWBCs. How to test if I'll regret disconnecting the neutral from an outlet? Will a clamp on amp meter show a reading over the neutral? – Mazura Aug 17 '14 at 20:26
  • @mazura "not in tandem", I meant not in double pole breakers. Also not necessarily adjacent either. – Mazura Aug 17 '14 at 21:34
  • @Mazura - you can nearly always shuffle breakers around in the panel until you can get both MWBC breakers adjacent to each other, then tie them together or swap out to a double-pole breaker. If you DON'T, what happens when some innocent electrician shuts off ONE breaker and unwires the neutral? That neutral will now be HOT through the load on the second branch. If she's got a dicky ticker, she might now be dead. Yep, an amp clamp will show current through a neutral IF the load is unbalanced. If balanced, no. – TDHofstetter Aug 17 '14 at 23:05
  • @user24125 - I'm having a bit of trouble finding the specification for matched breaker sizes. I know I've run across it, though... and it makes sense. Since all (including common neutral) conductors serving an MWBC are required to be the same size, then the breakers serving those wires also need to be the same size to match the wire size. – TDHofstetter Aug 17 '14 at 23:08
  • The amp meter should show you the imbalance between the circuits only. – user24125 Aug 17 '14 at 23:29
  • @TDHofstetter nothing requires matched breaker sizes on MWBCs. Matched wire sizes, perhaps, but you are always allowed to downsize a breaker. For instance if you convert an old dryer circuit to MWBC serving an RV via a NEMA 5-30, and a string of outlets in a garage (5-15 and 5-20). You must put a 20A breaker on the latter. It must be handle-tied with the 30A breaker unless you provide another disconnecting means. – Harper - Reinstate Monica May 19 '16 at 23:21
  • The requirement is for a disconnecting device which disconnects both sides of an MWBC together, to protect service personnel. There is no requirement that the sides trip together. So for instance if you used fuses, you would need to provide a gang switch that disconnects all poles in one motion. It would be weird if you had untied breakers and also a disconnecting switch. So I don't recommend that. – Harper - Reinstate Monica May 19 '16 at 23:25
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You need to identify each pair of hot wires with there corisponding neutral first try starting at the romex sheath in the panel with the power to the panel OFF and follow each to there termination and label them. And relocate breakers as necessary. If this is not possible it is much more difficult .If you can find the first box that each MWBS is run to you can use a breaker finder tracer and a continuity tester to locate each wire at the panel. Tracer for the hots and continuity for the neutral with the wire taken loose at both ends. Continuity test needs to be done with all power off. Continuity can be used for all wires and is easiest with 2 people and a 2way radio or cell phone. If you aren't sure of how to test continuity tell me and I'll explain.

user24125
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